In math class, the girl to boy ratio is $7$ to $8$. If there are $21$ girls, how many boys are there?
Solution: A ratio of $7$ girls to $8$ boys means that there is a group of $7$ girls for every group of $8$ boys. If there are $21$ girls, then there are $3$ groups of $7$ girls each. Because we know the ratio is $7:8$ , there must be $3$ groups of $8$ boys each. There is a total of $24$ boys in math class.